Luciferases are found in a wide variety of organisms including fireflies, photobacteria, jellyfish and many others. Luciferases are enzymes which catalyze the production of light by oxidizing luciferin to oxyluciferin in a process known generally as bioluminescence.
The production of photons by luciferase occurs through a two step reaction which consumes luciferin, adenosine triphosphate (ATP) and O.sub.2. In the first step, luciferase catalyzes the formation of luciferyl adenylate from luciferin and ATP. In this first step pyrophosphate is released and a Mg.sup.+2 cofactor or another divalent cation is required for proper luciferase function. Upon formation, luciferyl adenylate remains within the active site of luciferase. In the next step, luciferase oxidizes luciferyl adenylate to an electronically excited oxyluciferin with the consumption of oxygen. Light production occurs when the electronically excited oxyluciferin decays to the ground state oxyluciferin. The decay from the excited state to the ground state occurs with the concomitant emission of a photon. The color of the light produced differs with the source of the luciferase and appears to be determined by differences in the structure of the various luciferases.
Luciferases have recently become useful in reporter-gene technology. In this technique, a reporter gene, such as a luciferase encoding polynucleotide is used as an indicator for the transcription and translation of a gene in a cellular expression system. The reporter gene is operatively linked to a promoter that is recognized by the cellular expression system. Other commonly used reporter genes include .beta.-galactosidase, and chloramphenicol acetyltransferase (CAT). In a typical reporter gene assay, a DNA vector containing the reporter gene is transfected into a cell capable of expressing the reporter gene. After a sufficient amount of time to allow for the expression of the reporter gene has passed, the cellular membrane is disrupted to release the expressed gene product. The reagents necessary for the catalytic reaction of the reporter gene are then added to the reaction solution and the enzymatic activity of the reporter gene is determined. Alternatively, the cell can be disrupted in the presence of all reagents necessary for the determination of the enzymatic activity of the reporter gene. If a .beta.-galactosidase encoding polynucleotide is used as the reporter gene, the hydrolysis of a galactoside is determined. If a chloramphenicol acetyltransferase encoding polynucleotide is used as the reporter gene, the production of an acetylated chloramphenicol is determined. When luciferase is used as the reporter gene the photons produced from the luciferase-luciferin reaction is measured.
A major problem in determining expression of a luciferase gene as a reporter gene is the short duration of photon production. Typically, luciferase catalyzed photon production ceases within a few seconds. Means for extending the period of photon production have been eagerly sought. Currently a commercially available kit from the Promega Corporation (Madison, Wis.) can extend the half-life of luciferase catalyzed photon production to roughly five minutes. Nevertheless, for the measurement of large numbers of samples, luciferase catalyzed photon production with a half-life of only five minutes is not a viable alternative. As used herein, half-life is the time it takes for photon production to decrease by one half.
The present invention provides methods and compositions for increasing the duration of detectable photon emission of a luciferase-luciferin reaction.